The EPA has issued regulations for reducing the level of sulfur in gasoline and diesel fuel. In order to comply with these new regulations, essentially all domestic refineries are forced to install new fuel desulfurization processes. Well-known hydrotreating processes are commercially available. These hydrotreating processes operate at relatively high pressures and use significant amounts of hydrogen. Therefore, these processes require a significant capital investment and have high operating costs. In addition, most gasoline desulfurization processes based on hydrogenation degrade the quality of the gasoline.
In order to reduce the overall cost of desulfurization, several new technologies are being developed which do not utilize conventional hydrotreating technology. Two of the most recognized non-conventional approaches are adsorption of sulfur compounds onto a solid adsorbent and extraction of sulfur compounds into a immiscible liquid phase. Phillips Petroleum has developed technology (U.S. Pat. No. 6,274,031) which utilizes a fluidized bed for adsorption of the sulfur compounds. Regeneration is in an oxygen environment requiring lockhoppers. The process has yet to be commercialized but the capital costs are believed to be higher than conventional hydrotreating. Other technologies based on adsorption such as Pritchard's process (U.S. Pat. No. 5,730,860), have failed to commercialize to date at least partially due to operational concerns and capital cost projections.
A desulfurization technology based on liquid/liquid extraction of the sulfur compounds would be attractive since generally mild conditions would result in lower capital and operating costs. However, a suitable solvent has been difficult to find since the organosulfur compounds in a hydrocarbon mixture generally have physical properties similar to the other organic compounds. One approach is to increase the polarity of the organosulfur compounds by partially oxidizing them and then extracting them into a polar solvent. In U.S. Pat. No. 5,910,440, Grossman, et al. proposed a process to oxidize the sulfur species to sulfoxides and/or sulfones using a microorganism in an aqueous based system. The sulfoxides and/or sulfones are subsequently reduced by a reducing agent in the aqueous phase. U.S. Pat. No. 6,160,193 issued to Gore, also proposed a step-wise process for oxidizing the sulfur compounds using an oxidizing agent such as peroxyacetic acid followed by extraction with a non-miscible solvent such as dimethyl sulfoxide. This process is further explained in a paper, AM-00-25, presented by Petro Star, Inc. at the National Petrochemical and Refiners Association 2000 annual meeting. UniPure Corporation presented an additional paper at the 2001 National Petrochemical and Refiners Association annual meeting, AM-01-10, proposing a similar approach based on an aqueous oxidation. All of these processes utilize a step-wise oxidation/extraction approach. Since the partially oxidized sulfur compounds are not fully extracted by the solvent, a fixed bed adsorption step is required downstream of the extraction step. In addition, these processes consume expensive chemical reagents.
There are other novel technologies being developed. In U.S. Pat. No. 6,274,026, Schucker, et al. propose to polymerize sulfur compounds in an electrochemical cell using an ionic liquid as the electrolyte. The sulfur-containing polymers deposit in electrochemical cell making the separation difficult and resulting in an inefficient batch process.
The present invention described in detail below involves the use of ionic liquids to extract organosulfur compound. The organosulfur compounds may be extracted directly or they may be partially oxidized to sulfoxides and sulfones so as to increase their solubility in the ionic liquids. Ionic liquids are molten salts composed entirely of ions. Molten NaCl is a common example. When the cation is a relatively large organic cation and the anion is, for example, a metal halide, the melting point of the salt is lower such that it is a liquid at room temperature. Room temperature ionic liquids were developed in the 1970's and the early research took place in the U.S. focusing on the use of these materials in batteries. In the 1980's interest developed in using room temperature ionic liquids as solvents for chemical processes, and since then a number of such uses have emerged. Some research has focused on using room temperature ionic liquids in biphasic systems for alkylation and acetylation reactions, “Ionic liquids prove increasingly versatile”, Chemical & Engineering News, Jan. 4, 1999. Systems using room temperature ionic liquids as solvents to extract organics from aqueous solutions have been developed, “Room temperature ionic liquids as a novel media . . . ”, Chem. Commun., 1765–1766, 1998; “Green processing using ionic liquids and CO2, Chemical & Engineering News, May 10, 1999.
Institut Francais du Petrole holds over a dozen relevant patents using room temperature ionic liquidsas solvents for alkylation, polymerization, and diels-alder catalysts dating from the early 1990's. For example, U.S. Pat. No. 5,892,124 describes a process for diels-alder reaction in which a room temperature ionic liquid is used as a solvent for a Lewis acid catalyst. U.S Pat. No. 5,550,304 discloses a dimerization process using a quaternary ammonium halide combined with an aluminum halide and/or an alkylaluminum halide as the room temperature ionic liquid.
U.S. Pat. No. 5,304,615 assigned to BP Chemicals provides a butene polymerization using pyridinium or imidazolium chloride combined with an alkylaluminum halide, RnAlX3-n as the room temperature ionic liquid.
U.S. Pat. No. 5,824,832 assigned to Akzo Nobel describes a process for linear alkylbenzene formation using proprietary room temperature ionic liquids as described in U.S. Pat. No. 5,731,1091 that are made with alkyl amine hydrohalide salts combined with a metal halide.
U.S. Pat. No. 5,220,106 assigned to Exxon discloses a process using an ionic liquid to extract aromatics from a mixed hydrocarbon in which the preferred salt is triethylammonium dihydroxybenzoate. There remains a significant need to develop improved processes for removing sulfur from hydrocarbon materials and for developing novel and beneficial uses of ionic liquids.